The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having optical bistability.
Bistable optical elements have a plurality of stability points for each input level and are considered as basic elements to constitute circuits having such optical functions as optical logic, optical memory and optical delaying. Such optical functional circuits are usually composed by arranging many bistable optical elements in parallel. However, since known bistable optical elements, above all bistable lasers, are realized by forming multiple semiconductor layers formed over a semiconductor substrate and providing the elements with divided electrodes, its output light is usually parallel to the semiconductor substrate. (See for example, H. Kawaguchi, "Bistable Operation of Semiconductor Lasers by Optical Injection", ELECTRONICS LETTERS, Vol. 17, No. 20, October 1981, pp. 741-742, and Ch. Harder el al., "Bistability and Pulsations in CW Semiconductor Lasers with a Controlled Amount of Saturable Absorption", APPLIED PHYSICS LETTERS, Vol. 39, No. 5, September 1981, pp. 382-384.) As a result, where many elements are to be formed over the same substrate, they can only be arranged in a one-dimension or integrated only in a linear array, so that the number of elements that can be integrated is inevitably limited. As a solution to this problem, there is known the so-called surface emission laser, which generates laser oscillation in a direction normal to the substrate and the semiconductor layers laminated over it. This type laser enables the elements to be arranged in two dimensions over the substrate surface, or integrated in a matrix form, but the surface emission laser itself has so high an oscillation threshold as to make its operation not only extremely difficult at room temperature but also virtually impossible when integrated. Therefore, it is by no means a useful element for any practical purpose.